The present invention relates in general to a computer-based method for graphic reproduction, and in particular to a computer-based electronic page composition system.
Computers are frequently used to create print sets for layouts in magazines, brochures, and other printed material using color electronic pre-press systems. Images from a variety of sources including color scanners, digitizers, computer graphics, and text processing systems, can be integrated and viewed or modified on a video monitor. When the integrated page is finalized it is "rasterized," or encoded into a pixel-by-pixel format, and sent to an output device which records the image on film. Color images are usually produced using four color process half-tone screens. Therefore, one color page, made up of a variety of images, results in four half-tone screens, one for each of the four printing colors.
Pages of published material contain color images consisting of line art and continuous tone pictures, usually reproductions of photographs. Line art, including text and graphic objects, have different requirements for editing and reproduction than do continuous tone pictures. To reproduce line art without any blockiness or roughness in the edges, it is necessary to represent line art at a very high spatial resolution. Continuous tone pictures can be photomechanically reproduced at a relatively lower resolution, but require a large number of possible gray or color shades to be defined for every point in the reproduction. If the shades are not adequately defined, posterization, lack of smoothness, or other quantizing artifacts occurs.
Various methods have been developed to automate the production of color images. Scanners electro-optically scan color pictures and create half-tone color separations on high-contrast black and white photographic films at the correct sizes. Typesetters, using digitally encoded character data, create correctly sized and formed characters that are recorded on photographic film or paper.
Once the various image elements have been created and separately recorded on individual pieces of film, they must be combined into full page separations, or flats, before they can be printed. A full page separation is required for each color ink that will be used on the final printed page. The full page separations are created by stripping, or page assembly, a time-consuming manual process that involves cutting the various elements from their respective substrates and fastening them to carrier sheets, or flats, using adhesives. Each color separation of each element must be positioned on its respective flat in the correct position and in register with the other color separations. The required precision typically is to a small number of thousandths of an inch.
The full-page separations, one for each printing color, are used to make printing plates. The plates are mounted on a printing press and each is used to apply a different color ink to the paper, thus resulting in a full color print.
Some of these manual stripping operations have recently been automated. When image elements are represented in digital form in a computer, they can be electronically merged with other elements before being written on a single film using a phototypesetter, or an imagesetter. Scanned elements are represented in a computerized system as rasters, or matrices of pixels, and each pixel is represented by the intensity of each of its constituent colors, or by the amount of colorant to be applied.
Line art, such as text and graphic objects, is often represented using a vector format, with mathematical equations representing the boundaries of the lines and curves making up the image element, and various color codes describing the fill and border colors. With vector representation, text and graphic objects are resolution-independent until they are rasterized.
A raster image processor, or rasterizer, merges the scanned (raster) elements and the line art (text and graphic objects) elements on a page into a single composite raster image of the page containing all of the elements, each in its proper position. The resulting composite digital raster image is then recorded onto a sheet of photo-sensitive film using a phototypesetter, imagesetter, or other similar device.
Before rasterization, the vector data representing graphic objects and text is not usually stored in a computer using a raster or pixel-by-pixel format, but instead, because text and graphic objects are usually made up of geometric shapes of single colors and covering large areas relative to the size of a single pixel, they can be created and stored in a computer using mathematical descriptions rather than by individual definitions of each pixel. This allows text and graphic objects to be stored without using a large amount of memory.
Most film recorders are raster-oriented devices and record image data on film in a pixel-by-pixel and line-by-line sequence. Therefore, vector data that represents text and graphic objects cannot be recorded directly, but instead must first be "rasterized." Usually, a raster image processor transforms the mathematical descriptions of this line art into a raster format. The raster image processor (RIP) may be implemented either in software or hardware.
Unlike line art, raster images, such as digital representations of photographs, are stored in a computer pixel-by-pixel, each pixel having a defined color. The rasterizer takes this pixel-by-pixel data and produces new pixel-by-pixel data at the output resolution which may not be the same resolution as that of the original raster data. The rasterizer next merges all image elements and creates a representation of the page where the color of each pixel in the page is stored.
In the industry standard four color printing process four different color inks are printed. When printed, these primary colors appear to mix and give the appearance of many other colors. The last step of the rasterizer is to calculate the intensity of each primary color to use to give the correct color appearance of each pixel. The final result is four pages, or color separations, each representing the page in a different one of these primary colors.
In present systems, when raster images such as photographs are rasterized, they are scaled to the resolution of all of the other graphic objects so that they can be rasterized at the correct resolution. This change of resolution may be performed by the raster image processor, or by a preprocessor. Raster images, although stored at a lower resolution, require much more memory space than text or graphics require because they are represented in pixel-by-pixel format rather than by mathematical descriptions.
Line art, such as text or graphic objects, must normally be printed at a much higher resolution than is necessary for continuous tone images so that output images will have the desired sharpness, clarity and crisp line definition, and will not have jagged edges.
Using existing automated production methods, when raster images are combined with line art, the raster images must be rasterized at a high resolution, usually the resolution of the line art elements. Typically, a resolution from one million to over five million pixels per square inch is used. Because the raster images must be represented at this resolution, a great deal of computer memory is required. In addition, the processing time greatly increases when raster images are processed at the high resolution because such a large number of data elements must be processed.
Almost all high resolution film recorders are raster (line by line) oriented devices. Therefore, to produce high resolution film from artwork necessitates rasterizing the artwork when either color film recorders (2000, 4000 or 8000 pixels per line) or monochrome typeset film recorders (1000 to 5000 pixels per inch) are used. Producing film for photomechanical reproduction also involves separating and transforming colors into four or more inks, and requires a film to be produced for each color.
When photographic images are combined with text or graphics on a single page, three types of techniques are currently used to create a page layout that includes both types of images. The first is to create two or more separate sets of films where the text images are at a high resolution and the photographic images are at a lower resolution. The films are then physically cut and pasted (stripped) to integrate the two into a single image to be printed.
The second technique currently used involves assembling and screening an image page, including text, graphics and/or raster images, in software using one resolution for the entire image. If the page, and therefore the raster image, is rasterized at a high resolution, this technique requires much more processing time than would otherwise be necessary. For example, an 8 by 10 inch image would require 1 to 2 hours to process each of the four ink separations, resulting in a total processing time of 4 to 8 hours. In addition, much more memory space is used to store the photographic image at the higher resolution than is needed to store it at the lower resolution.
A third technique involves assembling the screened page in hardware using more than 100 Megabytes of data memory. This approach is expensive, and requires a large amount of special purpose hardware. These systems are generally used only for black and white images.
It is therefore an object of the present invention to provide a technique for merging images which have been rasterized at different resolutions.
It is a further object of the present invention to provide a technique for merging images without increasing the amount of storage needed to produce the integrated image.
It is also an object of the present invention to provide a technique for merging images without increasing the amount of time that is needed to produce the integrated image.
It is an additional object of the present invention to provide a technique for merging images using a system that requires decreased cost and complexity than present systems.
It is a further object of the present invention to provide a technique for merging images without physically cutting and pasting separate images.